FIG. 2. (a) Confocal scan showing the fluorescence spots of the shallow-implanted N-V− centers on the front facet of the sample. The leftmost waveguide (marked with a green box) is studied in the following. (b) Mode shape of the waveguide (WG). The shape of the WG mode is revealed by laterally scanning a 738 nm laser while recording the collected counts in transmission. The two intersections at the side show the corresponding 2D Gaussian fit to the WG mode. The corresponding mode-polarization contrast is shown in the lower right corner. Note that the image is rotated by 90◦ with respect to panel (a). (c) N-V− transmission through the waveguide. We excite the N-V− centers off-resonantly with 532 nm laser light from the front and collect the transmitted signal. Laterally scanning the excitation laser resembles the waveguide mode in transmission. The intersection of the dashed lines denotes the position which is at 61% of the maximum of the WG mode (green ellipse). (d) Experiment schematic. The sketch illustrates the two detection channels, reflection and transmission. (e) The PL spectra in reflection and transmission are compared. The exposure time of the transmission measurement is ten times larger than for the measurement in reflection. Both spectra are normalized to the ZPL. The inset shows the ZPL polarization measured in transmission through the waveguide, which resembles the waveguide mode polarization (marked as a green line). (f) We extract the wavelength-specific optical detection efficiency of the N-V− ensemble to the waveguide mode by comparing the transmitted PL spectrum with the fluorescence in reflection and by accounting for losses that occur until detection. The green curve is the inferred efficiency for a N-V− ensemble located at 61% of the optimal coupling corresponding to the green ellipse in (c). The black lines marks the 1/e2-profile of the Gaussian mode.
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